In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting material has been actively developed. In particular, a light-emitting element called an electroluminescence (EL) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered applicable to a light source such as a backlight of a liquid crystal display and an illumination device.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. Specifically, by applying a voltage with a light-emitting layer provided between electrodes, electrons and holes injected from the electrodes recombine to raise the light-emitting substance to an excited state, and light is emitted when the substance in the excited state returns to the ground state. There are two types of the excited states which are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. In contrast, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as a fluorescent compound), in general, phosphorescence cannot observed at room temperature, and only fluorescence can be observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
In contrast, the use of a phosphorescent compound can increase the internal quantum efficiency to 100% in theory. In other words, emission efficiency can be 4 times as much as that of the fluorescent compound. For these reasons, in order to obtain a highly efficient light-emitting element, a light-emitting element using a phosphorescent compound has been developed actively recently. As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal have particularly attracted attention because of their high phosphorescence quantum yield (for example, see Patent Documents 1 to 4).
Specifically, Patent Document 4 discloses an organometallic complex that has a 4-arylpyrimidine derivative as a ligand and iridium as a central metal.